It is common in today's switch-based telephone networks to offer call redirection services such as call forwarding and call distribution, usually on a subscription basis. When a subscriber having a default DN (directory number) wants to activate call forwarding for example, the subscriber enters a forwarding DN, and a signalling procedure takes place between the subscriber's telephone and a network node which may be a telephone network switch, or an AIN (advanced intelligent network) based SSP (service switching point) for example. The network node then controls the redirection of calls initially directed towards the subscriber's default DN such that they are received at the forwarding DN. Such a centralized mechanism for providing features to subscribers requires a substantial investment in operations, administration, and maintenance functions.
The use of computing devices connected to an IP (internet protocol), ISDN (integrated serviced digital network) or ATM (asynchronous transfer mode) based LAN (local area network) or WAN (wide area network)) to assume multimedia communications roles has recently become increasingly widespread. Examples of this include voice communications and video conferencing communications over private intranets or the public Internet. It is desirable to be able to offer users of such multimedia communications media call redirection features which are similar to those which are available to normal telephone subscribers. For example, when an end-user having a computer normally connected to a default location with a normal address within a particular LAN moves to a different known location where he/she has access to another computer having a different address, it would be useful to be able to have multimedia calls placed to his/her normal address forwarded to the different address. A dedicated computing device or server may be used to provide the required call redirection capability to a number of end-users. This "server based" approach requires related operations, administration and maintenance functionality, requires address translation and related signalling from the end-user's computing device to the dedicated computing device, all of which adds complexity which may not be desirable from an administrative authority's perspective. In addition, having the call redirection functionality on a dedicated device offers limited end-user control and customization over how his/her calls are to be forwarded.
The Mobile IP Protocol (defined by the mobile IP work group within the IETF (Internet Engineering Task Force)) is an existing IP (Network) layer procedure for terminal mobility within the Internet/Intranet [Perkins, C., Editor, "IP Mobility Support", RFC 2002, October 1996]. This protocol however, requires the modification of a mobile host's IP stack to handle IP encapsulation or tunnelling, the modification of the home router's IP stack to support the functionalities of a Home Agent (i.e. encapsulating/decapsulating and tunnelling both call signalling and media packets to the destination c/o address), the existence of a Foreign Agent on the foreign network (router) to handle IP encapsulation/decapsulation and tunnelling at the far end, procedures to discover and register with Home Agents and Foreign Agents, and procedures to gain access to firewalls when the callee's mobile host is attached to a subnetwork behind a firewall.
Although comprehensive and currently gaining wide acceptance, the Mobile IP protocol requires modification to both the home network router's, foreign network router's and the mobile host's IP stack before being applied to realize terminal mobility. It provides very little end user control over how his/her calls are to be redirected, since call redirection is performed at the IP layer within the router.